1. Field of Invention
This invention relates to heating heat softenable sheet material and, in particular, to heating glass sheets that are conveyed on conveyor rolls through a sheet heating furnace.
2. Technical Considerations
Shaped and tempered glass sheets are widely used as windows in vehicles such as automobiles and the like. It is important that the windows meet stringent optical requirements and be free of optical defects that would tend to interfere with clear viewing through the window or detract from the window's appearance. Commercial production of such shaped glass sheets commonly includes the steps of serially conveying the glass sheets through a tunnel-type furnace where they are heated to their heat deformation temperature, and thereafter conveying the heat softened sheets into a shaping station where they are shaped by any of a number of well known shaping processes, e.g. as disclosed in U.S. Pat. No. 4,139,359 to Johnson, et al.; U.S. Pat. No. 4,666,492 to Thimons, et al.; U.S. Pat. No. 4,767,434 to Schwartz et al.; U.S. Pat. No. 4,496,386 to Hymore, et al. and U.S. Pat. No. 4,830,650 to Kelly. After shaping, each glass sheet is transferred to a cooling station for controlled cooling.
To heat the glass sheets, typically high powered radiant electric coils are positioned within a heating furnace along the furnace's ceiling and floor so that the glass sheets are heated from above and below. Because of the difficulty in balancing the amount of heat absorbed by the glass from the top heaters as compared to the bottom heaters, oftentimes it is difficult to keep the glass sheet flat during its initial heating. More specifically, the heat energy from the top heaters penetrates and heats the glass sheet while the conveyor rolls tend to block and absorb a majority of the heat energy directed at the glass sheet from the bottom heaters, which, in turn, increases the temperature of the conveying rolls. The rolls then heat the glass sheet by direct contact along the bottom glass surface. However, this condition tends to heat the sheet only along its surface rather than through its thickness so that the glass sheet must rely on heat conduction through the sheet to move the heat upward into its core. Since glass is a poor conductor of heat, this method of heating the bottom surface and lower portions of the sheet is inefficient. The heating imbalance results in a temperature gradient through the sheet such that there is a higher temperature in the upper portion and top surface of the sheet as compared to the lower portion and bottom surface of the sheet, which, in turn, expands the top surface more than the bottom surface and causes the glass sheet to dome upward. When doming occurs, lead edges, corners and/or points of the sheet tend to move downward below the general plane of support provided by the conveyor rolls. As the glass moves through the furnace, these portions of the sheet contact the rolls below the support plane, which may result in a scraping action between the roll surface and the glass surface. In an effort to reduce the thermal gradient and resulting doming effect, the amount of heat directed at the sheet from below the conveyor rolls may be increased but this may result in simply increasing the temperature of the conveyor rolls.
It would be advantageous to provide a heating arrangement to efficiently heat a glass sheet in a manner that reduces any temperature differential between the top and bottom surfaces of the sheet and any thermal gradient through the sheet's thickness so as to reduce glass sheet doming and associated surface marking.